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title The syscall of the wild

The Syscall of the wild

Compile me with make README.html

  • Diff between function call and syscall?
  • test.c and test.s
  • asm("nop; nop;"); creates a "bookmark" in the asm output.
  • How to clobber memory? asm(:: "memory");

Function Call

mov    $0xe,%ecx
lea    0x0(%rip),%rbx # 18 <main+0x13>
mov    %rbx,%rdx
mov    $0x1,%esi
mov    $0x1,%edi
call   2a <main+0x25>

Syscall

mov    $0x1,%eax
mov    $0xe,%edx
mov    %eax,%edi
mov    %rbx,%rsi
syscall

Why do we need syscalls?

  • Security

Libc

vDSO

  • Why is switching to kernel-space slow?
  • Is it necessary all the time?
  • Enter: vDSO

vDSO Security

  • Return-to-libc
  • ASLR

Calling the old vdso "UNSAFE" as a config option is just plain stupid. It's a politicized name, with no good reason except for your political agenda. And when I call it out as such, you just spout the same tired old security nonsense.

Tiny binaries

  • Use raw syscalls -> no need for libc.

  • 

    • 



attribute((force_align_arg_pointer))
void _start() {
...
__builtin_unreachable();
}



- also `-strip` and `-Os`